The present invention relates generally to fiber-reinforced composites wherein the matrix consists of a ceramic material. More particularly the invention relates to ceramic matrix composites comprising reinforcing inorganic fibers having a carbon coating disposed thereon.
Fiber-reinforced ceramic matrix composites comprising glass and/or glass-ceramic matrices are well known. U.S. Pat. No. 4,626,515 for example, discloses fiber-reinforced composites wherein the reinforcing fibers are composed of silicon carbide and the reinforced matrix is an essentially alkali-free, alkaline earth aluminosilicate glass. U.S. Pat. Nos. 4,615,987, 4,589,900 and 4,755,489 disclose silicon carbide fiber reinforced glass-ceramic composites wherein the matrix materials consist of an alkaline earth aluminosilicate glass-ceramic composition.
Proposed applications for ceramic matrix composites include components for heat engines. Such components must exhibit good strength and toughness at ambient and elevated temperatures in an oxidizing environment.
Composites reinforced with silicon carbide fibers such as Nicalon.RTM. fibers exhibit good strength and toughness, in part due to the presence of a graphitic layer at the fiber-matrix interface which controls fiber-matrix bonding and improves crack-resistant behavior. However, relatively high composite processing temperatures, typically above 1300.degree. C., are presently needed to promote the development of this graphitic layer in the composite. This high temperature processing is expensive and can also harm other components of the composite.
Fiber coatings have been proposed as an alternative way to control fiber-matrix bonding. U.S. Pat. No. 4,397,901, for example, describes a composite article and method for making it wherein the fibers are provided with successive coatings of pyrolytic carbon, diffused silicon, and silicon carbide. U.S. Pat. No. 4,405,685 describes a similar coating system for carbon fibers wherein an inner coating consisting of a mixture of carbon and a selected metal carbide is used. U.S. Pat. Nos. 4,376,803 and 4,376,804 describe composite coatings consisting of amorphous carbon with overlayers of a metal oxide such as SiO.sub.2 for reinforcing metal alloys.
Notwithstanding the development of these coatings, no effective functional replacement for the prior art graphitic interface has yet been discovered. Thus in silicon carbide fiber-reinforced ceramic matrix composites wherein this layer has not been adequately developed, brittle rather than tough failure modes are generally demonstrated.
Thus there remains a need for a simple coating system, and an economical method for applying it, which would protect inorganic fibers to be utilized for the reinforcement of ceramic matrix materials while preserving the desirable toughening and reinforcing characteristics of those fibers.
Particularly useful would be a functional replacement for the prior art graphitic interface which could be provided by a simple, low-temperature coating process. Such a process would reduce the potential for damaging the fibers at high temperatures.
Most preferably, chemical vapor deposition processing as used for fiber coating in the prior art would be avoided, as this is complex and expensive.
It is therefore a principal object of the present invention to provide a method for coating inorganic fibers which improves the utility of the fibers for ceramic matrix reinforcement.
It is a further object of the invention to provide coated silicon carbide fibers which can be used to manufacture fiber-reinforced ceramic matrix composites at relatively low and thus more economical temperatures.
Other objects and advantages of the invention will become apparent from the following description thereof.